If you are evaluating a Verify API, you are usually trying to protect three things at the same time.
- Your conversion rate during signup and login
- Your fraud exposure during high-risk actions
- Your team’s time spent fighting delivery issues, bot abuse, and user complaints
A Verify API can look similar across providers on the surface. You send an OTP, the user types it in, you get a success or fail.
In production, the differences show up in the parts you only notice after launch. Delivery speed varies by country. Some routes get filtered. Fraudsters spam your endpoint with OTP requests. Users in certain markets ignore SMS but respond quickly on messaging apps. Your support team gets screenshots of “I never got the code” all day.
In this guide, we will look at:
- What to look for in a Verify API
- How multi-channel fallback should work
- How pricing is charged when fallback happens
- The security and reporting features you actually need
What a Verify API does in a CPaaS setup
A Verify API is typically built on top of a CPaaS (Communications Platform as a Service) and an A2P (Application to Person) messaging platform. In a basic OTP flow, you could call an SMS API directly. A Verify API adds orchestration and control.
Common Verify API features include:
- OTP generation and validation
- Delivery across one or more channels
- Fallback rules when a message is not delivered
- Rate limiting and anti-abuse controls
- Webhooks, logs, and analytics
When your verification traffic becomes meaningful, orchestration becomes the product. The channel is just the transport layer.
That is also where A2P messaging matters. Verification traffic is A2P messaging by definition. It is triggered by your app, it needs consistent deliverability, and it is sensitive to filtering and fraud controls.
Juniper Research estimates total operator business messaging revenue at $51.7 billion in 2025, growing to $54.6 billion by 2030, which gives you a sense of how large A2P services are in the telecom economy.
Businesses add verification because account takeover and scam-driven losses are huge, and phone verification is still one of the most widely deployed friction points. The FBI’s Internet Crime Complaint Center reported 859,532 complaints and $16.6 billion in losses in 2024, a 33% increase from 2023.
Verification is not a silver bullet. It is a layer that can reduce abuse, reduce account takeover, and support stronger risk decisions. Your Verify API choice affects whether that layer helps conversion or harms it.
Start With a Tight Requirements List
Before you look at vendors, define your requirements in a way that your team can verify with tests.
Define your verification events
List the exact events you will protect in the first release:
- New account registration
- Login from a new device
- Password reset
- Adding a new payout method
- High-value transaction approval
- Changing a phone number
Treat each event as a different risk tier. Your requirements can differ by tier.
Define what success means
Pick target numbers for the metrics that matter to you:
- Verification completion rate by country
- Average time to first code delivery
- Share of sessions that require fallback
- Support tickets per 10,000 verification attempts
- OTP resend rate
Even if you do not know the right target yet, choosing the metrics gives you a clean way to compare providers.
Define your channel policy
Decide upfront if you want a single channel or multiple channels.
Using SMS or voice over the public telephone network to deliver one-time codes should be treated as a higher-risk method, not a default. When you rely on it, you need stronger checks around the phone number and device, including signals like SIM swaps, device changes, and number porting, so suspicious activity triggers extra verification or step-up security.
That does not mean you must avoid SMS. It means your plan should include:
- Alternatives for users who cannot receive SMS reliably
- Risk checks for number changes and suspicious behaviour
- A clear path for step-up security for high-risk actions
Multi-channel verification is one practical response because it gives you more delivery options while keeping your UX familiar.
The Checklist That Separates a Basic OTP API From a Verify API You Can Run Long-term
Below is a high-intent evaluation checklist. You can use it to build your procurement scorecard.
Channel coverage and fallback behaviour
A Verify API should be able to deliver codes on the channels your users actually open.
SMS.to’s Verify API is explicitly built around multi-channel delivery across SMS, Viber, WhatsApp, and Telegram, with rules for fallback order and timeouts.
When you evaluate any Verify API, confirm these details:
What channels exist for your target markets
Ask the provider to show:
- Channel availability by country
- Any country restrictions for sender types
- Any onboarding steps required for non-SMS channels
If you operate in multiple regions, test a few representative countries rather than assuming global behaviour is consistent.
How fallback works when delivery does not happen
You want the provider to explain fallback with specific rules, not marketing language. You are looking for:
- What triggers fallback
- How long each channel waits before switching
- Whether retries happen on the same channel before switching
- Whether the provider can stop after a successful delivery event
SMS.to has real-time orchestration and automatic fallback across channels until delivered or timed out” with a default configuration stored in one Verify App and per-request overrides via API.
A practical test you can run during evaluation:
- Set primary channel to a channel you know is not installed on the test device
- Confirm fallback happens within your expected time window
- Verify you only receive one usable OTP message, not duplicates
Pricing model and what you actually pay for
Verification costs often surprise teams because you pay for two things:
- Delivery cost of the message
- Verification orchestration cost
You want a pricing model that is easy to forecast.
With SMS.to Verify API for instance, you only pay for what actually gets through. If your OTP is delivered on WhatsApp, Viber, or Telegram, you’re charged just that channel’s fee plus the Verify fee. If the flow falls back to SMS, the SMS is charged when it’s sent, and the Verify fee applies only if the message is delivered. You’re not billed for skipped channels or for delivery attempts that fail, and each OTP is billed once for a single delivery channel, with the Verify fee charged only on successful delivery.
When you evaluate providers, look at the pricing based on your real flow, such as:
- 1,000,000 verification attempts per month
- 70% delivered on the first channel
- 20% require fallback
- 10% fail after all channels
Cost controls you should ask for
- Caps and alerts for daily verification volume
- Per-country pricing visibility
- Separate fees for attempts versus verified outcomes
- A way to block traffic if an abuse spike happens
Delivery telemetry and operational visibility
If you cannot see what happened, your team will be groping in the dark. And that creates support tickets and product debates that never end.
SMS.to’s Verify API includes delivery and verification telemetry, webhooks and searchable logs, as well as statistics and charts. You even get a Verifications area and breakdown charts for monthly verification statuses and fees.
When you evaluate a Verify API, look for these:
Webhooks you can rely on
You want:
- Delivery status callbacks for each step
- Verification status callbacks
- Retry behaviour that is clearly documented
- Enough identifiers to match events to your user session
Logs you can query without exporting everything
That way you can filter by:
- phone number
- time range
- status
- channel
- application
This matters most when you are debugging a market-specific problem.
Metrics that help product decisions
Good analytics help you answer:
- Which countries rely heavily on fallback
- Which channels complete verification fastest
- What time-to-code looks like across networks
- Where resend requests spike
That feeds directly into conversion improvements.
Security and anti-abuse controls
Verification endpoints get abused. You will see:
- OTP spam attempts
- credential stuffing flows followed by OTP prompts
- bots trying to enumerate valid phone numbers
- fraud attempts using SIM swaps and porting
In the U.S., SIM-swap takeovers are common enough to show up as a standalone crime type: the FBI logged $25,983,946 in reported losses from such incidents in 2024. In the UK, unauthorised SIM swaps jumped to nearly 3,000 cases in that same year (up from 289 in 2023) and that’s happening alongside the everyday churn of legitimate SIM replacements for lost phones, upgrades, and eSIM moves.
You do not need that exact pattern in every market to see the point: number ownership can change, and verification flows should treat changes as higher risk.
Controls you should demand in a Verify API evaluation
- Rate limiting per phone number and per IP
- Resend limits and cooldown windows
- Ability to set expiry time per request
- Fraud signals around number changes where possible
- A clear policy for rejecting VOIP numbers if that matters to your risk model
Email and VOIP endpoints are not acceptable for out-of-band authentication because they can be received by more than one endpoint, weakening the possession proof.
Webhook security and request integrity
If your webhook endpoint is public, attackers can post fake events to it. That can lead to incorrect verification states in your system.
SMS.to offers signed webhook events using the X-SMSto-Signature header, so you can verify events are from SMS.to and not altered in transit. Each endpoint gets its own webhook secret.
During evaluation, confirm:
- How signatures are generated and verified
- How secrets are stored and rotated
- What happens if signature verification fails
- Whether replay protection exists or can be built with timestamps and ids
Setup time and team workload
Procurement teams often focus on contract terms. With CTOs, the priority is security and reliability. For product teams, eyes are on conversion.
You also need to focus on how quickly you can ship.
With SMS.to, go to Verify API → Request Activation in your dashboard, then configure your Verify App settings (OTP format, expiry, retries, and a callback URL) and make sure your account has a balance so support can activate it. Once SMS.to’s specialists set it up, you’ll see a Verifications submenu where you can track results in the UI or receive them automatically via your callback URL
For your evaluation checklist, ask:
- What is the average time from signup to production-ready Verify API use
- What onboarding steps exist for each channel
- What you can configure yourself and what requires support involvement
- What SDKs or code samples exist for your stack
Scorecard You Can Use in Procurement
Use a simple 1 to 5 score per category and write down evidence from your tests.
Deliverability and user experience
- Time to first message in key markets
- Share of sessions requiring resend
- Fallback works as configured
- Users receive one clear message, not duplicates
Security and fraud controls
- Rate limiting and resend rules
- Webhook signature support
- Secret management and access controls
- Guidance for handling number change risk
Analytics and troubleshooting
- Searchable logs
- Webhook event detail
- Export options
- Charts that support product improvements
Pricing predictability
- Clear fees for message delivery and verification
- Transparent charging rules for fallback
- Alerts and caps
Compliance posture
If you send SMS for verification and later expand into messaging, it helps to align with messaging best practices early.
CTIA’s Messaging Principles and Best Practices say you should tell people exactly how to opt out using standard “STOP” instructions, but also treat normal-language replies like “end,” “unsubscribe,” “cancel,” “quit,” or “please opt me out” as valid, even if the user changes capitalisation or punctuation.
When someone opts out, you should send one final opt-out confirmation per campaign and then stop messaging them. You also need to keep auditable opt-in and opt-out records (for example, timestamp, method, what the user saw/did, the campaign, and the phone number) and regularly remove deactivated numbers so you do not message dead lines.
Verification traffic is not marketing, but you still want internal discipline around consent, records, and user communication. It reduces friction when you expand into broader A2P messaging later.
Examples for Testing the Verify API
Testing with real flows is better than reading feature lists.
Signup verification in two regions
Test the same flow for two countries you care about.
- Country A where SMS is reliable
- Country B where users rely more on messaging apps
Your evaluation target:
- 90% or higher completion rate for signup verification
- Median time-to-code under 10 seconds for the primary channel where possible
- Fallback triggers correctly and does not confuse the user
Password reset under attack pressure
Simulate a spike in reset requests.
Your evaluation target:
- Rate limiting blocks abuse patterns
- Legitimate users still complete resets
- Logs show you which requests were blocked and why
High-risk action step-up
Example actions:
- changing a payout account
- changing a phone number
- increasing transaction limits
Your evaluation target:
- You can shorten expiry time
- You can add additional checks for number change events
- You can force a higher-confidence channel order when needed
Questions to Ask Vendors
The answers you get on these will help you narrow down your choices:
Coverage and routing
- Which channels are available in our top 10 countries?
- What sender types are supported per country?
- What is the fallback logic and what triggers it?
- What happens if a channel is temporarily unavailable?
Security and compliance
- Do you support signed webhooks and how do we validate them?
- What controls exist for rate limiting and abuse prevention?
- What guidance do you have for number change risk and SIM swap exposure?
Observability
- What identifiers exist to match events to sessions?
- What logs are searchable in the dashboard?
- What is the webhook retry strategy?
- What reporting exists for conversion and cost?
Billing clarity
- What fees are charged on attempts versus verified outcomes?
- How are failed deliveries treated?
- How do we forecast costs for fallback-heavy markets?
Getting A Solution That Works For You
A Verify API is a product surface. Your users see it when they are trying to get into their account or complete an action they care about. That is not the place for flaky delivery or confusing retries.
- Pick the provider that performs best in your top markets under realistic tests.
- Pick the pricing model your finance team can forecast.
- Pick the webhook and logging capabilities your engineers can support during incidents,
If you want a solution that is built around multi-channel delivery, rules-driven orchestration, detailed telemetry, and a clear charging model for fallback, SMS.to’s Verify API is designed for exactly that set of needs.
